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SQL & Relational Databases Interview Questions & Answers

Joins, indexing, ACID transactions, normalization, query optimization, and triggers.

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How do INNER JOIN and LEFT JOIN differ when a customer has no orders?

An INNER JOIN returns customer-order combinations only when the join condition matches. A LEFT JOIN retains every customer from the left side and uses NULL for order columns when no match exists. Plac...

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How should SQL queries test for NULL, and why is NULL = NULL not true?

NULL represents missing or unknown information, and ordinary comparisons involving NULL evaluate to unknown rather than true or false. Use IS NULL or IS NOT NULL to test for it. WHERE keeps only rows ...

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